# Homework Help: Physics of Car Crashes HELP

1. Nov 18, 2004

### bubblez

Physics of Car Crashes HELP PLZ :)

hello!
okay ive got an assignment due tomorrow and would very much like some help please!

is this right:
crumple zones absrob energy that is supposed to be transmitted to the passenger and therefore because it absrobs the energy the passenger does not usffer such a huge shock when the car crashes???

whats a dependent and controlled variable?

and is umpulse and momentum the same?

thanks so much!

2. Nov 18, 2004

### Raza

all i can say that this thing is right
"crumple zones absrob energy that is supposed to be transmitted to the passenger and therefore because it absrobs the energy the passenger does not usffer such a huge shock when the car crashes???"

3. Nov 18, 2004

### MR

a dependent and controlled variable in the context of the rest of your post, or just in general?

As far as impulse and momentum go: they are not the same thing, but they do have to do with each other.

Momentum is the mass of something multiplied by its velocity.
So if you're falling and you want your momentum at impact you multiply your mass times your velocity when you hit the ground.

Impulse is the change in momentum divided by the change in time.
So when you hit the ground after your fall, your momentum goes from something-or-another to zero (because you stop moving in the end). You can adjust how much the fall will hurt by bending your knees when you fall, giving yourself a longer time to go from yes-momentum to no-momentum.

I hope that helps a little. I think I should've used numbers and letters (it really does make more sense that way!) but they scare a lot of people, so I opted for words instead. Sorry!

4. Nov 18, 2004

### Galileo

Suppose a car drives against a wall which brings it to a grinding halt.
The total change in kinetic energy is the same whether you have a crumple zone or not (the impulse $\int F(t)dt$ is the same).
What a crumple zone accomplished is that the momentum changes more or less gruadually (recall that dp/dt equals force).
If dp/dt is low, you won't feel so great a shock.
You will get thrown forward with the speed you were going before banging against the wall, but because of the safety belts, you'll be kept in your chair. The force the belts apply to your body are lowest when the contact with the wall is as long as possible. (Crumple zones really don't help if you don't wear safety belts).

So I think it's wrong to say the crumple zone absorbs energy which would otherwise be transmitted to the passenger.

Last edited: Nov 18, 2004
5. Nov 18, 2004

### QuantumTheory

Let dp/dt equal the change the rate of change of momentum over time.

Then,

You could say that $$\int\lim_{dp\rightarrow dt} = 0.$$

6. Nov 18, 2004

### QuantumTheory

Ok, I'll try to explain it better.

There is something called inertia, momentum is carried by inertia. Inertia is the resistance and will for something to move until an outer force stops it. That basically means something will move if some force effects it, and something will only stop if some force stops (i.e friction, gravity, etc.)

When you drive a car down the road, with a seal belt or not, there is a great deal of momentum involved due to the speed. The more massive the object in motion the greater the momentum.

Remember, for every action there is an equal and opposite reaction.

So, I'm driving my car without a seat belt on at 50 MPH, suddenly I slam the breaks on to avoid a crash and come to a halt. What happens is your body continues to move at the speed of the car when you slammed on the breaks.

The only difference is when you wear a seat belt it holds you back, otherwise, you'd go straight though the window!

Remember, you're going 50 MPH, so if you slam the breaks on, the forces will be violent since you're going from 50 MPH to 0. Why? Because it took an amount of force and time to accelerate you to 50 MPH, what comes in comes out. Therefore It will take the same amount of force to stop you, and when you stop so suddenly, those forces can't work as well because it took you longer to get to 50 MPH then it did to stop.

Last edited: Nov 18, 2004